Millimeter wave rotational spectrum of HCNO in vibrationally excited states

Abstract

A millimeter wave spectrometer equipped with a free space cell and on-line computer averaging has been employed to measure the absorption spectrum of H 12C 14N 16O in the frequency range from 40 to 300 GHz. For each rotational transition up to J = 11 → 12, all of the l-components of transitions in the following vibrational states have been observed and analyzed: 0000 02 0, 2, 0000 03 1, 3, 0002 0, 20 0, and 000(11) 0, 2. The method of analysis, carried out on the basis of the Nielsen-Amat formulation of the rotation-vibration interaction in linear molecules, is discussed. The unperturbed rotational constants B v and D v as well as several vibration-rotation constants were determined using sum rules valid for each group of perturbed levels and lines. The vibrational anharmonicity constants g l t l t′ were determined for the first time from pure rotational transitions. The following constants were obtained: gl 5l 5(v 5=2)=−24.2±3.0 cm −1, gl 4l 4(v 4=2)1.28±0.30 cm −1 gl 5l 5(v 5=3)=−6.24±0.14 cm −1, gl 4l 5(v 5=1; v 5=1)=1.63±0.50 cm −1; An interpretation of the results of this analysis, especially for the v 5 = 2 and v 5 = 3 states, is given. The observed anomalies indicate the presence of a potential function with a hump disturbing the isotropic two-dimensional oscillator potential for ν 5, the low-lying degenerate bending mode. In the case of the combination state 000(11) 0, 2 the vibrational l-type doubling constants r 45 (0) = −0.81 ± 1.00 cm −1 and r 45 (1) = 0.6565 ± 0.0027 MHz have been evaluated for the first time from millimeter wave rotational transitions.